A DAYLIGHT AND HEAT GAIN 1) HEAT GAIN is the amount of heat that gets into a space FROM the outside in summer conditions when air conditioning is desired. Air conditioning is simply removal of heat from a space. 2) HEAT LOSS is the opposite. The amount of heat that gets away from a space TO the outside when comfort heating is desired. Comfort heating is the process of adding heat to a space.

3) Two kinds of heat gain: BY CONDUCTION through the various materials of the building envelope when heat is transferred from molecule to molecule. The number of BTUs that travel through one square foot in one hour per degree Fahrenheit temperature difference is called a “U” factor. BY RADIATION of electromagnetic energy directly through glass – which does not become heat energy until sufficient mass changes EM energy to heat.

DIRECT SUNLIGHT DIRECT SUNLIGHT a) Sunlight is the source of electromagnetic energy which in turn becomes a source of heat, but not until b) The rays strike a surface of sufficient density to convert to heat. c) Sunlight increases the temperature of a concrete walkway much higher than ambient temperature, because of its density, but does not for surfaces such as grass and soft earth.

d) An adjacent grassy lawn surface may seem cool to the touch, because of the difference in the density. e) The softer surface acts to absorb the sun’s rays with too little density to produce heat. f) Rays of the sun may penetrate glass but not become heat until they contact a dense surface inside, such as a floor, wall, furniture, people, etc. Glass transforms some EME to heat, but not much, because of its thinness.

g) Then, electromagnetic energy produces heat that becomes a cooling load during air conditioning seasons. h) If glass has a mirrored surface such as a coating sufficient to reflect direct sunlight, then the source of heat is diminished. i) But there is a tradeoff. A surface that can reflect light also may diminish light transmissibility.

SHADING DEVICES SHADING DEVICES a) A shading device may be anything that keeps direct sunlight from striking a surface – or from penetrating a surface to produce heat on the other side. b) Shading may be a tree, an overhang, a directional louver, a window drape or blind. c) Glass itself can be a shading device if it is tinted to reduce transmissibility of sunlight, or if coated with a reflective surface to deflect heat-producing rays.

d) Not many shading methods allow free use of daylight, except for strategically placed mechanical shades that protects only in areas needed. e) Well designed landscape is useful on east and west exposures when sun angles are low. f) Landscape may be ineffective on south exposures since the south is subject to direct light most of the day, and sun angles vary.

g) Landscape plants of deciduous variety are particularly useful, since in summer they shade, but in winter with no leaves they allow heat producing sunlight. h) Shading devices on high rise buildings are impractical, which may be limited to the use of reflective coatings on transparent surfaces.

SHADING COEFFICIENTS SHADING COEFFICIENTS a) Refer to supplementary material for charts that contain shading data. b) A shading coefficient is a number between zero and one, that represents the value of a shading device in diminishing the effect of direct rays of sunlight. c) It is a percentage of direct sun, hence the smaller the number, the better the material as a shading device. d) Shading coefficients are used to modify the calculation of the amount of heat that gets through glass into a space from measured affects of direct sunlight at specific times of the year called “solar gain.”

B HEAT GAIN THROUGH WINDOWS 1) HEAT FLOW THROUGH GLASS a) Heat caused by radiation is technically not ‘heat flow’, since the heat is not transformed until it is inside the space – by connecting with a dense surface. b) Direct heat travels by conduction, from warm to cool. Conduction tends to balance both sides of a barrier by heating the material and flowing through.

c) Conduction of heat is transferred from molecule to molecule of a material, and if unabated will continue until the entire material is the same temperature as the ambient air. d) The difference in temperature between outside and inside is the force that causes heat flow through a building envelope. e) Regardless of shading devices, coated glass, or landscaping, heat will travel through glass simply because of the difference in temperature between the inside and outside surfaces.

f) Tinted Glass, also called “heat absorbing glass” was developed because dark colors absorb heat. The premise was that glass in sunlight would absorb heat and the surrounding air would dissipate the heat before it penetrated inside. The process is effective slightly. INSULATING GLASS a ) Consider two panes of glass held apart with edges sealed together, and dry air in the space between panes.

b) Dry air prevents condensation from forming inside the space between the panes because no moisture is present there. c) Resistance to heat flow is doubled with two panes of glass and the air space adds an insulating value. d) Insulating glass with two panes and one air space is a little more than twice as good at resisting heat flow by conduction as a single pane of glass.

e) Insulating glass made with clear panes does nothing to prevent heat caused by radiation from entering a space, because radiation passes through and is not heat until transformed by striking a dense surface on the other side. f) Formerly, insulating glass was very expensive, had to be ordered in exact dimensions, and took a long time to get – because the panes had to be made by fusing the edges of glass together. g) Today, insulating glass is made by putting two or more panes together with a metal strip at the perimeter edges between the panes and fastening with epoxy glue – exhausting the air in the space and replacing it with dry air.

REFLECTIVE GLASS REFLECTIVE GLASS a) Since insulating glass does not stop radiation of direct sunlight, a reflective coating is placed on one side of an insulating glass panel. b) The process is much the same as coating a glass to make a mirror, except it is not opaque.

c) Coatings on glass are subject to deterioration due to exposure to weather and cleaning, so the coating must be placed inside the insulating air space. d) Coatings range in appearance from light to dark in a variety of colors, and in a range of effectiveness to REFLECT radiation penetration by 70% to 20%.

1” INSULATING GLASS

C SOLAR GAIN AND DIRECTION ORIENTATION OF BUILDINGS a) Orientation is related to sun direction – not only east – west, but sun angle height at worst conditions. b) East or west sides are good at selective times for horizontal daylight – effective if shading is not prohibitive. c) South exposure is worst for buildings above 23 degrees latitude since that direction is vulnerable to radiant energy the whole day.

d) North exposure is best for buildings above 21 degrees, since direct sun exposure is minimum. e) North exposure is best for daylighting and best for minimal heat gain due to sun radiation. f) North exposure is NOT the worst condition for winter heating conditions, because HEAT LOSS from a space to the outside is mostly by conduction. g) Ambient temperature difference is the only consideration for temperature for winter heating conditions – radiation is not a factor.

SOLAR GAIN SOLAR GAIN a) Solar gain is the amount of heat that is produced as the result of direct radiation from the sun. It is measured in terms of the number of BTU per hour per square foot of glass that is subject to direct radiation from the sun – diminished only by a shading coefficient b) Solar gain varies in its intensity at different times of the day. Obviously when the sun’s rays extend through a maximum of atmosphere, the affects of radiation are filtered. c) Solar gain also varies at different times of the year, because of the tilt of the earth relative to the sun.

Since heat gain in cooling season can be diminished by preventing radiant energy from the sun, the orientation, color, and reflectance of the building envelope must address the issue. Because of the difference in ambient temperature from inside to outside, the composition of the building envelope must maintain an integrity to reasonably reduce the gain or loss of heat due to conduction.

HEAT FLOW VARIATION BY DAY AND BY SEASON AHeat flow in winter is caused by the difference in temperature on both sides of the barrier. With the exception of ventilation and infiltration, heat flow is by conduction. BAccumulation of heat within a space in summer is caused by a combination of things; Conduction through opaque surfaces Conduction through transparent surfaces Radiation through transparent and opaque surfaces * Heat from infiltration Heat from outside make-up air due to ventilation

CSince the sun seems to trek across the sky during the day from east to west, the intrusion of radiant energy varies at opposite times on the east and west. DWith sites above 23 degrees north latitude, radiant energy is not a major factor in solar gain on the north side. EFor any site above 23 degrees north latitude, the south exposure is subject to direct sun radiation during most of the day. FThe earth rotates (day / night), and also tilts on its axis relative to the sun by an amount of 46 degrees and 42 minutes. (summer / winter)

FBecause of the tilt, any point on the earth’s surface is at a varying distance from the sun, and at a varying angle from the horizon. GConsider in Lubbock, Texas at 34 degrees N, when the sun is at its highest peak in the summer, which means the sun’s position would be at 23 degrees and 21 minutes north of the equator. The sun would be at its closest point to Lubbock. Summer solstice, the most severe radiant energy. HBut then the sun (so it seems) moves toward the south, passing the equator at the autumnal equinox, then on south of the equator an angle of 23 degrees 21 minutes south. IAt that point the sun is at its farthest distance from Lubbock, (winter time) and at its lowest angle with the horizon from Lubbock. Winter solstice, the least amount of radiant energy.

The calculation for the maximum amount of heat gain for a space happens during the period of the summer solstice. The calculation of heat gain is affected by two major considerations, both caused by the varying position of the sun. Equivalent Temperature Difference (ETD), which takes into account the density of mass of a building envelope (to retain or expel heat), the outside color of the envelope (to reflect or to absorb heat), and the time lag during the day when part of the heat absorbed by the envelope escapes into the space. Solar Gain, which is the amount of heat that gets into a space by the inclusion of electromagnetic energy through translucent and transparent surfaces.

HEAT FLOW BY CONDUCTION AND RESISTANCE TO HEAT FLOW AObviously, heat flow must have a scale of units. Heat is measured in terms of BTUs per hour, and measured in quantity through a material or assembly of materials in BTUs per hour, per square foot of surface area, per degree Fahrenheit difference in temperature between the inside and outside of the space. BBTU is an acronym for British Thermal Unit, which is the amount of heat required to raise one pound of water one degree Fahrenheit.

CAll building materials are laboratory tested to determine their rate of heat flow by conduction, which is their measure to conduct heat. DA material has a Conductance factor, designated C, which is the amount of heat that will flow through a given standard thickness, such as a 1” thick board, or a 3 5/8” thick brick. EA material has a resistance to heat flow, designated as R, which is its measure of its value as an insulator. FA material has by the Inch-pound system of measure a rate of conductivity, designated as k, which is the amount of heat flow through the material per inch of thickness.

GSince conductance and conductivity is heat flow through a material by conduction, and resistance opposes heat flow, it follows that conduction and resistance are reciprocals, so there is a mathematical relationship. C = 1 / R, and R = 1 / C C = k / inches thickness, and k = inches thickness x C R = inches thickness / k, and k = inches thickness / R WhereC is conductance k is conductivity k is conductivity R is resistance

CONVECTION Convection is simply the movement of warm air within a space For the simple reason that warm air is less dense than cold air (molecules are further apart), warm air rises in an uneven temperature and cold air falls. That is the reason why hot-air balloons rise in the atmosphere. The air in the balloon is warmer than the air outside – hence the mass of the balloon is less dense because the air is warmer. Considering only density, that is why a stick of wood held under water will rise to the surface when released; the wood is less dense than water so the denser medium forces it upward. Convection is not a factor of heat flow.

Air Film At the exposed surfaces of solids, heat transfer takes place by conduction and by radiation. When air motion along surfaces is minimal, an insulating layer of air is created – and heat transfer through the layer is by conduction When air motion is increased, such as outside with wind, the insulation value decreases.

EMITTANCE – the energy radiated by the surface of a body per second, per unit area. Materials with low emittance radiate less heat. Reflective materials have low emittance. Emittance is a factor in choosing the conductance and resistance value of air film, inside (still air) and outside (moving air). Table 4.3, of the text shows resistance values of still air and moving air films, adjacent to a building envelope. Column, Inch-Pound units, non reflective For still air (inside), vertical surface, R = 0.68 for emittance of.90 For moving air (outside), 15 mph wind, R = 0.17 Which means air film has insulating value - but not much. Which means air film has insulating value - but not much.

INSULATING MATERIALS Most all building materials have some insulating value, but remain of secondary importance. Wood siding and brick is selected for appearance, finish, durability – not for insulating value. In comparison: material “R” value material “R” value 3 5/8” thickness brick /8” thickness brick.56 ¾” wood board.75 ¾” wood board.75 ¾” polyfoam board 4.68 ¾” polyfoam board ½” fiberglass insulation ½” fiberglass insulation 11.00

Examples of Conversion of Resistance and Flow: 1 If a material has a C value of 0.12, what is its R value? Since R = 1/C, then R = 1 /.12 = 8.33 Since R = 1/C, then R = 1 /.12 = If a material has a k value (heat flow per inch of thickness) 0.15, what is the R value for a material that is 4 ½” thick? Since R = inches thickness / k, then R = 4.5 /.15 = 30 R = 4.5 /.15 = 30 3 If a material 2 ½” thick has an R value of 16, what is its k value ? Since R = inches thickness / k, then k = inches thickness / R, and k = 2.5 / 16 =.156. k = inches thickness / R, and k = 2.5 / 16 = If a material 6” thick has a k value of 0,156, what is the C value? 4 If a material 6” thick has a k value of 0,156, what is the C value? Since C = k / inches thickness, then C =.156 / 6 = Since C = k / inches thickness, then C =.156 / 6 = 0.026

Remember that insulation does not stop heat flow... It only slows it over time. A building in a cold climate left vacant for long periods of time that has water pipes inside must be left with a source of heat. If not, the water will eventually freeze and the pipes will burst. The only thing that stops heat flow (by conduction) is no medium, which is to say, a vacuum.

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